I did some conversions as Craig said:
dbexport
of Informix database- some search & replace in schema text file (I use Python for such things).
- on PostgreSQL server:
psql -f [schema_file] new_database
- analyze
psql
output and if something is wrong drop new PostgreSQL database, and correct search & replace tools (goto 2)
When you convert all interesting things into PostgreSQL you can compare both schemas using my tools like: http://code.activestate.com/recipes/576621-dump-informix-schema-to-text/
Those tools are used mainly to compare Informix vs other Informix or PostgreSQL vs other PostgreSQL database but after some normalizing I believe you can compare PostgreSQL vs Informix.